Spectrometry using continuous wave (CW) tunable sources with narrow spectral linewidth and long coherence lengths has well-known advantages associated with high spectral contrast, frequency selectivity and excellent sensitivity. Scanning CW terahertz (THz) spectrometers are a prime example of this technology. In such systems, phase stability in the transmitter-to-receiver demodulation processing may be required to obtain an accurate measurement of the transmitted electric-field intensity and to characterize any resulting absorption losses from samples in the spectrometer. However, the signal generated by a THz spectrometer may contain a strong background signature created by superimposed standing wave patterns within the spectrometer cavity or sample cell.
As the path length of the spectrometer increases, the variation in amplitude as a function of measurement wavelength may also increase in frequency. A spectrometer acquiring data at a discrete set of THz frequencies, then may under sample the background pattern. The resulting background signature may be an aliasing of the actual standing wave structure within the instrument, and change shape and amplitude as the measurement frequency sampling is altered.
In the presence of small frequency calibration drifts occurring in a scanning spectrometer, the sensitivity of the background structure to frequency sampling may cause changes in that structure over multiple measurements. This variation in the background between scans may reduce the effectiveness of a simple ratio technique—dividing a measurement by a reference taken with no sample present—to accurately determine the spectral features of a sample being tested.